In conventional door structures of automobiles, a door inner panel is provided with an opening formed for work and a door hole cover which covers the opening in order to prevent water from penetrating from the opening.
However, in such door structure, due to vibration of the door inner panel or pneumatic vibration caused by vibration of a vehicular body or by vibration from a door speaker, the door hole cover itself generates vibration sound (abnormal sound) such as fluttering or chattering sounds.
In order to suppress such vibration sound, it has been proposed, for example, to partially form a plurality of beads which expand in their thickness directions, on an opposite section facing an opening in the door hole cover (see, for example, the following Patent Document 1). In Patent Document 1, the rigidity of the opposite section is improved, to thereby suppress the vibration sound generated from the bead formed portions.
Further, in order to suppress an interference sound generated when the vibrating door hole cover interferes with the door inner panel, it has been proposed, for example, to partially laminate a damping material which is made of foamed rubber material based on EPDM, on a surface of an opposite section facing an opening in the door hole cover (see, for example, the following Patent Document 2).    [Patent Document 1] Japanese Unexamined Patent Publication No. 2007-290684    [Patent Document 2] Japanese Unexamined Patent Publication No. 2008-12993